Schwendig said a bomb's shock wave alone was often enough to rupture blood vessels in victims' lungs and other organs in a way that proves fatal, and that debris picked up by explosions – fences, barricades, wood – tends to fly at people, severing limbs. Shrapnel packed into bombs is made to pepper victims with nails and metal ball bearings, severing veins and arteries.

"It's just one more way to hurt people," Schwendig said of the shrapnel Boston doctors said was built into the bombs. "A nail that is tumbling through the body – that nail is an-inch-and-a-half long and it's tumbling and it's going to cut right through that artery."

Still, most of the ball bearings and other pieces of shrapnel will probably remain with the victims for the rest of their lives, he said. Removing the fragments could cause more damage and increase the likelihood of infection, so trauma doctors often leave the shrapnel where it is.

Many victims of the Boston Marathon bombings face possible amputations in the coming days and months, hospitals reported.

Brigham and Women's Hospital has already performed one amputation, and is working to save another patient's injured limb. Massachusetts General Hospital has amputated four limbs so far, and is treating two patients who could face amputation in the coming days. At Tufts Medical Center, doctors have not yet performed any full amputations, but four victims have limb-threatening injuries.

Dr. Alex Jahangir, an orthopedic trauma surgeon at Vanderbilt University Medical Center in Nashville, Tenn., said blast wounds were rare but cause severe, vast injuries. He said he rarely performs amputations within the first day of an injury because it can take 24 hours to determine the full extent of the damage.

Instead, Jahangir performs an initial surgery simply to clean the wound and remove dead bone and muscle. He may later perform an amputation, depending on whether the limb is getting enough blood flow, whether it has enough healthy tissue and whether there's an infection present.

Sometimes, the patient has a choice between amputation and salvaging a limb, Jahangir said. But many times patients choose amputation because the recovery is faster.

Amputees are usually fitted with prosthetics and sent to rehabilitation to learn how to use them within six to eight weeks, Jahangir said. Patients who don't chose amputation often undergo 15 or 20 surgeries, and they may need to wait six to eight months to make sure the bone is healed before they can start walking.

"That process takes two to three years, and at the end of that, they still may wish that'd have gotten it amputated," Jahangir said. "You'll have the leg, but it won't be the same leg you had."

To help patients decide between keeping a limb and amputating it, he calls in Donna Bogg, 50, who nearly lost her leg in a car accident when she was 22 and chose to amputate four years later because of pain and complications. Bogg answers patients' questions about life as an amputee when doctors don't know how to answer, she said.

In Massachusetts, the New England Amputee Association hopes to do the same for the Boston Marathon victims, said its founder, Rose Bissonnette, 66. She lost her left leg 16 years ago when she was crushed under a tractor trailer and had to decide whether to undergo amputation.

"I wanted to speak to a woman amputee," she said. "I didn't have that, and that's why we're here today. ... Our part is to listen to the patients and their families. We've been through this."

Dave Covino, 38, of New Hampshire has visited with patients facing amputation over the years, and he said they always ask about their limitations.

"I had my amputation when I was 16 years old, and my three questions were: Can I still drive a stick shift? Will I be able to go to prom? And can I ski again?" he said.

Covino went to his prom on crutches, still has his right foot to hit the clutch pedal on a stick shift and clocked his top skiing speed at 52 miles per hour last season.

"So all in all, even though I went through a lot of life changes, it was not the end of the world," he said.